Carbon nanotube technology is quickly becoming a technological area that is making an impact on the field of microelectronic devices. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
Single-wall carbon nanotubes are quasi one-dimensional nanowires, which exhibit either metallic or semiconducting properties, depending upon their chirality and radius. Single-wall nanotubes have been demonstrated as both semiconducting layers in thin film transistors as well as metallic interconnects between metal layers.
One technology uses carbon nanotubes as an electromechanical switch for non-volatile memory devices, where the nanotubes are spin-deposited over a patterned substrate surface. The nanotubes 12 lay over trenches 14 between a first electrode 16 and a second electrode 18 of an integrated circuit 10, as depicted in FIG. 1. The device 20 is switched on by applying a bias to the second electrode 18, and switched off by removing the bias to the second electrode 18, and applying a bias to the first electrode 16.
A two-terminal switching device 20 can be made by over-lapping a metal layer over a nanotube layer 12, as depicted in FIG. 2, where the metal layer is segmented into a first electrode 16 and a second electrode 18. By applying a voltage to the nanotube layer 18, a nanoscale space or cavity 14 is melted between the overlapping portions of the nanotube layer 12 and the second electrode 18, which cavity 14 becomes the switching region for the switch 20. The cavity 14 allows the distal end of the nanotube layer 12 to move freely to perform switching function.
In this method, however, the formation of the cavity 14 is not a controlled process. For example, the variation in the density of the carbon nanotube layer under the second electrode 18 can by itself lead to a wide variation in the various sizes of the cavities 14 that are formed in integrated circuits from across a substrate. Because the height of the cavity 14 determines the switching voltage of the device 20, the switching characteristics can therefore vary greatly across the substrate. Furthermore, there is no limitation in the amount of the metal that forms the second electrode 18 that can be consumed during the melting process. Continued operation of the switch 20 may result in a total consumption of the second electrode 18, and thus reduce the operation lifetime of the switch 20.
What is needed, therefore, are alternate designs for nanotube switch structures, and alternate methods for the fabrication thereof.